When CNG (Compressed Natural Gas) is used as a fuel, oil dispersed from, for example, a compressor during compression of natural gas is sometimes admixed as oil mist to the CNG. If the CNG containing the oil mist is supplied to an engine, the combustion efficiency is decreased. Therefore, it is necessary to remove the oil mist from the gas (CNG).
Furthermore, if combustion gas (blow-by gas) leaking from a combustion chamber of an engine is emitted to the atmosphere, it causes environmental pollution. For this reason, this gas is circulated again to the combustion chamber with a blow-by gas circulation unit and combusted. However, engine oil used for engine lubrication is dispersed, e.g., from cylinders and admixed as oil mist to the blow-by gas. Directly supplying such blow-by gas containing oil mist causes problems.
For this reason, a gas filter has been suggested for separating and removing oil mist from a gas such as the above-described CNG and blow-by gas (for example, Japanese Patent Application Laid-open No. 2001-120932).
In such a gas filter, a high separation (filtration) efficiency has to be implemented upon adjusting, for example, pressure loss of the filter element according to the properties of the gas which is to be filtered. However, with the conventional products, such an adjustment was difficult, and sufficient separation efficiency of oil mist could not be obtained for certain types of gases.